Before being delivered to the mixing apparatus of an asphalt plant, the aggregate materials are typically contained in a plurality of cold feed bins; each bin containing a particular grade of aggregate or used asphalt-aggregate material. The aggregate materials are then dispensed from the bins by a plurality of variable speed belt feeders, one of which is disposed below each bin. The belt feeders deposit material from each bin onto a conveyor belt for delivery to the mixing apparatus. Varying the speed of the belt feeder varies the rate at which the aggregate material is dispensed from the bins. However, it is necessary to know the actual rate at which aggregate material is dispensed from each bin to the conveyor belt in order to maintain a desired mixture of the various grades of aggregate material. The individual feed rates of the bins and the total feed rate for all the bins must also be accurately known in order to determine the precise quantity of liquid asphalt to be dispensed into the mixing drum to produce the desired asphalt-aggregate product.
U.S. Pat. No. 4,089,509 (hereby incorporated by reference) discloses a system for determining the individual and total feed rates of a plurality of cold feed bins. Each bin is equipped with a variable speed belt feeder whose rate may be controlled by a supervisory control system. The belt feeders dispense the aggregate material onto a conveyor belt disposed below the belt feeders. The conveyor belt transports the aggregate material away from the bins to the mixing drum of a drum mix plant where the aggregate material is processed into the desired finished product. Disposed below the conveyor belt are a plurality of weigh scales. Each scale is disposed slightly downstream of the point at which aggregate material is deposited onto the conveyor belt. As the belt passes over the weigh scales, the scales weigh the belt and the material disposed thereon. Comparing the weight registered by the various weigh scales permits calculation of the weight of material dispensed from the individual bins.
This prior art system has a number of disadvantages and inaccuracies inherent therein. The weight of the conveyor belt is seldom uniform throughout its length. This nonuniformity induces error into the weight measured by weigh scales underlying the conveyor belt. Furthermore, the weight of the conveyor belt will vary greatly, for example, if the belt gets wet from rain. The tension on the belt necessary to run the belt, when compared to the weight of the aggregate material disposed thereon, is great. Thus, small variations in belt tension tend to induce relatively large errors in measured weights of the aggregate material. Moreover, the tension of the belt often varies due to slip of the belt on the end and drive rollers. These variations in tension also induce error into the measured weight of the material on the belt.